This 13 us from physical discomfort and sickness.
答案：(C) shields
shields us from（保護我們免受……）表示這個修復系統保護我們免於身體不適與生病；conceals（隱藏）、derives（衍生）、transforms（轉變）均不符合此處語意。
shield 這個字，你的印象大概只有名詞的「盾牌」，遊戲裡拿來擋攻擊的那一面。它當動詞用的時候意思很直接——像盾牌一樣擋在前面，也就是保護、遮擋，而且幾乎固定與 from 一起出現。
判斷的關鍵不在空格本身，在空格後面那個 from。這一格要的是能走「動詞 ＋ 受詞 ＋ from ＋ 所要擋掉的東西」這條路的動詞，而且語意上要說得通：This ___ us from physical discomfort and sickness.（這使我們免於身體不適與生病）。國中學過的 protect ... from 就是同一個結構，shield 是它比較有畫面的說法。遇到動詞題，先看一眼空格後面的介系詞，常常一步就把範圍縮到剩一兩個。
conceal 雖然也能接 from，但 conceal sth from sb 是「對某人隱瞞某事」，代進去會變成這件事對我們隱瞞了不適與生病，語意不通。derive 確實常與 from 連用，但 derive from 是「源自」，主詞要是被衍生出來的那個東西，而本句主詞 This 指的是前一句的修復機制，它並不是從不適與生病衍生出來的。transform 配的是 into（transform A into B），不配 from。鑑別度高達 65，分開的是有沒有把 from 當成線索去反推動詞，還是只把四個動詞的中文意思擺進去憑感覺挑。
shield sb from sth（保護某人免於……）、protect sb from sth（保護某人免於……）、conceal sth from sb（對某人隱瞞某事）、derive from（源自）、transform A into B（把 A 轉變成 B）、physical discomfort（身體上的不適）
We all know that too much stress is not good for our health, but too little is not ideal, either. While 11 stress can be dangerous to the body, short-term stress is actually healthy.
Short-term stress triggers the production of protective chemicals in our body and strengthens the body's defenses. 12 our body is in a vulnerable situation, a burst of stress will quickly mobilize the body's own repair system to defend the damaged areas. This 13 us from physical discomfort and sickness.
Small amounts of stress hormones may even sharpen our memory. A recent study found that when rats were forced to swim—an activity that places them under stress for a short while—they remembered their way through mazes far 14 than rats that were in a relaxed state.
The key to a healthy lifestyle is to keep our stress level 15 . Too much stress will make us cranky and sick. Too little stress, on the other hand, will lead to boredom and low motivation.